Corn (maize) is one of the most diverse grain crops present in nature, comprising a number of different types, which are generally classified by characteristics of their kernel endosperm. The most common types of corn include flint, flour, dent, pop, sweet, waxy and pod. The physical appearance of each kernel type is determined by its endosperm pattern, quality and quantity.
Sweet corn is a corn plant classified as Zea mays, var. rugosa, and has white, yellow or bi-colored kernels that are sweet when they are in the immature milky stage as a result of having a high sugar content (i.e., sucrose content). Higher levels of sugar or sucrose in the sweet corn kernels result in a lower osmotic potential, causing greater water uptake into the kernels. Sweet corn is typically eaten by human beings as a vegetable, either directly from the maize cob, or by having the sweet kernels removed from the cob, and is a major vegetable crop grown all over the world primarily for fresh consumption, rather than as animal feed or for flour production.
Sweet corn occurs as a spontaneous mutation in field corn and can be the result of naturally-occurring mutations in one or more genes that control conversion of sucrose to starch inside the endosperm of the corn kernel. Unlike field corn varieties, which are intended for livestock and are typically harvested when the kernels are dry and fully mature (at the dent stage), sweet corn is typically picked when it is immature (at the milk stage), and eaten as a vegetable, rather than as a grain. Because the process of maturation involves converting sucrose into starch, sweet corn typically stores poorly and must be eaten in a fresh, canned or frozen manner before the kernels become tough and/or starchy. Following harvest, or if left on the stalk too long, sucrose in standard sweet corn becomes rapidly converted to starch. Kernels can lose as much as 50% of their sucrose at room temperature at around 24 hours after harvest.
Open pollinated (non-hybrid) varieties of white sweet corn started to become widely available in the United States in the 19th century. Two of the most enduring varieties, which are still available today, are Country Gentleman (a Shoepeg corn with small, white kernels in irregular rows) and Stowell's Evergreen. Sweet corn production in the 20th century was influenced by the following key developments: (i) hybridization, which allowed for more uniform maturity, improved quality and disease resistance; and (ii) identification of separate gene mutations responsible for sweetness in corn, and the ability to breed varieties based on these characteristics, for example: su1 (sugary); se1 (sugary enhanced); and sh2 (shrunken-2). There are currently hundreds of varieties of sweet corn, with more varieties continuously being developed.
There are several known genetic mutations that are responsible for the increase in sucrose content of sweet corn. Early varieties were the result of the mutant su1 (sugary-1) allele. Conventional su1 varieties contain about 5-10% sugar by weight.
Varieties of sweet corn that contain the shrunken-2 (sh2) gene typically produce higher than normal levels of sugar and have a longer shelf life, in comparison with conventional sweet corn, and are frequently referred to as supersweet varieties.
One specific gene in sweet corn, the shrunken-2 (sh2) gene, causes the mature corn kernel to dry and shrivel as it matures past the milky stage, which is an undesirable trait for seedling germination, early emergence and plant growth. The endosperm of conventional sh2 sweet corn kernels stores less amounts of starch, and from about 4 to about 10 times more sugar, than conventional su1 sweet corn. This has permitted the long-distance shipping of sweet corn, and has enabled manufacturers to can sweet corn without adding extra sugar or salt to it.
The third gene mutation is the se1 (sugary enhanced-1) allele, which is incorporated in the genome of Everlasting Heritage varieties. Conventional sweet corn varieties with the se1 alleles typically have a longer storage life, and contain from about 12% to about 20% sugar (i.e., a much higher sugar level in comparison with the conventional su1 varieties). The sugary enhanced allele is frequently referred to as the enhanced sugary trait or phenotype.
All of the alleles that are responsible for sweet corn are recessive, so the alleles described above must be in a homozygous state in order for the phenotype that they confer to the sweet corn plant to be observed.
Maize was first classified according to the carbohydrate that is stored in its endosperm. The most distinguishable sugar component that is present in sweet corn is sucrose, which accounts for the vast majority of its sweetness differentiation (Abbott and Cobb, Inc., Plant Protection No. 9600094 (1998)). The reducing free sugars, glucose and fructose, are present in sweet corn in significantly lower levels. These reducing sugars primarily result from the natural hydrolysis of sucrose.